An inkjet recording method has become rapidly popular in these days because it can record color images on plain paper and its running cost is low. However, the inkjet recording method has problems that image defects typified by character feathering (hereinafter, referred to as feathering) easily occur depending on the combination of an ink with a recording medium used, causing significant degradation in image quality. Thus, an attempt has been made such that ink permeability is suppressed so as to prevent feathering. However, in this case, since the drying properties of an ink degrade, hands are stained by the ink of a recorded matter and an image smear occurs.
When a color image is recorded by the inkjet recording method, different color inks are sequentially superimposed one another. As a result, color inks bleed and are mixed together at the boundary parts of colors (hereinafter, referred to as color bleeding), and image quality significantly degrades. To solve these problems, attempts have been made to prevent the color bleeding by increasing ink permeability. However, in this case, since a colorant penetrates inside the recording medium, image density degrades, and ink significantly strikes through to the back surface of the recording medium, failing to perform double face printing in a suitable manner.
To solve these problems and improve image quality, image forming methods using a pre-treatment liquid and an ink have been proposed.
Patent Literature 1 discloses a method of forming a coloring part on a recording medium using a liquid composition containing fine particles in a dispersion state, wherein the surfaces of the particles are charged with a polarity opposite to that of an aqueous ink. This method definitely effects on aggregation and adsorption of a colorant, but is unable to achieve a sufficient effect on the suppression of feathering, and color bleeding. Moreover, in this method, another problem occurs in terms of fixing ability of an aqueous ink. Specifically, the liquid composition and the aqueous ink are attached to the recording medium and then a vehicle (a liquid component in the liquid composition or aqueous ink) permeates the recording medium, thereby accumulating a mixture of the fine particles with the colorant on a surface of the recording medium. The thus formed accumulation is mechanically vulnerable, and may be easily removed by rubbing by hand and the like. Therefore, ink smears a user's hand and backface of stacked printed matter, resulting in failure of poor rubfastness.
Patent Literature 2 discloses an inkjet recording method in which a first liquid containing polymer fine particles and an ink composition are attached onto a recording medium to perform printing, in order to improving rubfastness of an image formed matter. According to the method, a large amount of polymer particles is necessary to obtain a sufficient rubfastness. However, the addition of a large amount of polymer particles may cause side effects, such as decrease in ejection stability and storage stability, viscosity increase or the like. Thus, problems have not been solved yet.
Patent Literature 3 discloses an inkjet liquid composition and an inkjet inkset, wherein the inkjet liquid composition contains an organic acid having an acid dissociation constant pKa of 4.5 or lower, an organic amine compound, an aqueous solvent and water, and a ratio of the amount of the organic acid and the amount the organic amine compound is specified, in order to improve storage stability of a pre-treatment liquid. Use of this inkset enables a color material of the ink to be effectively aggregated, since pH is maintained in an acid when the inkjet liquid composition and the ink are mixed. However, although the disclosed liquid composition achieves to attain storage stability, the disclosed inkset does not achieve less smear adhesion (also referred to as excellent rubfastness) of images immediately after printing since a colorant is aggregated and accumulates on a surface of a recording medium.
Thus, in the case where suppression of both feathering and color bleeding on an image formed matter is attempted to achieve by the conventional image forming method using the pre-treatment liquid in combination with the ink, the storage stability of the pre-treatment liquid degrades and the smear adhesion of the resulting images degrades.